mod_cache: Invalidate cached entities in response to RFC2616 Section

13.10 Invalidation After Updates or Deletions. PR 15868

Resolves outstanding issue with r1070179 as per
http://www.gossamer-threads.com/lists/apache/dev/395830?do=post_view_threaded#395830


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1478140 13f79535-47bb-0310-9956-ffa450edef68